1. Field of Invention
The present invention relates to a secondary battery protection device that detects an excessive current and shuts off a power supply circuit of a secondary battery.
2. Description of Related Art
FIG. 3 shows an example of a related art secondary battery protection device.
FIG. 3 shows a secondary battery 1, a charger 2, a switching element 3, such as an FET, a resistance to detect an excessive current 4, and a load resistance 5. These circuit elements compose a power supply circuit 6. A battery protection circuit 10 is equipped with a sense amplifier 7 that is connected to a positive side of the secondary battery 1, a sense amplifier 8 that is connected to a downstream side of the resistance 4, and an OR circuit 9 that takes an OR of outputs of the sense amplifiers 7 and 8 to turn off the switching element 3. The sense amplifier 8 connects to a battery 16 to provide a reference voltage. The battery protection circuit 10 is independent of a microcomputer, and exchanges signals with a CPU 11 of the microcomputer through a signal line.
Operations are described below. A voltage on the positive side of the secondary battery 1 is always detected by the sense amplifier 7, and a current that flows through the resistance 4 is detected by the sense amplifier 8. When an excess current is generated in the power supply circuit 6 for the secondary battery 1, an output that indicates an abnormality is generated in a voltage detection device by the sense amplifier 7 or a current detection device by the sense amplifier 8, the switching element 3 is turned off by an output from the OR circuit 9 to shut off the power supply circuit 6, such that the secondary battery 1 is protected.
However, a circuit, such as the one described above in which an output of the voltage detection means 7 or the current detection means 8 is directly inputted in the battery protection circuit 10, is subject to a problem. If a current or a voltage that is inputted in the battery protection circuit 10 is close to a threshold value, an output of the battery protection circuit 10 becomes unstable, such that the switching element 3 frequently turns on and off, which leads to a trouble in protecting the secondary battery 1.
Furthermore, since the battery protection device 6 is independent of the microcomputer, their circuit space becomes large and the size of the apparatus becomes large.
The present invention addresses the above and/or other problems, and provides a secondary battery protection device that enables miniaturization through building a battery protection circuit in a microcomputer.
The present invention can also provide a secondary battery protection device in which a power supply circuit does not frequently turn on and off even when a current or a voltage inputted in a battery protection circuit is close to a threshold value.